The present invention relates to a doctor element backing lath and doctor element assembly.
In the coating of paper and paperboard webs, to the surface of a moving web of paper is applied a layer of a coating mix, whereupon the coat is smoothed and the coating layer is doctored to a specified thickness by means of a doctor blade mounted on a support beam. The web to be coated passes through the nip formed between the doctor blade and a backing roll, whereby the blade doctors the excess coating off from the web surface and levels the remaining coating on the web surface into a layer of desired thickness. The doctor blade is loaded by means of backing lath adapted to rest against the blade either stiffly or flexibly. In a stiff loading assembly, the backing lath is adapted to rest directly on the blade, while in flexible loading there is adapted a flexible hose between the doctor blade and the backing lath.
To make the coating layer uniformly thick over its entire profile, the lineal loading force that pushes the doctor blade against the running web should be uniform over the entire width of the doctor blade. The doctor blade is loaded by moving the doctor blade support beam toward the backing roll, whereby the blade is compressed against the running web and bends about the loading line formed by the backing lath. Additionally, the doctor blade can be loaded locally by means of profile control screws that are adapted to effect on the backing lath and are placed over the cross-machine width of the web, typically at a distance of 45 to 150 mm from each other. The profile control screws make it possible to compensate, among other things, for defects in the base paper so that the defects will not be reflected on the profile of the coated web.
In most applications, the doctor blade may be replaced by a leveling rod assembly, wherein a rotating rod is used in lieu of a doctor blade. The rod of the leveling rod assembly is mounted on a holder having a flexible loading hose adapted thereagainst. Onto the other side of the loading hose is adapted a backing lath whose other side rests against the profile control screws.
Both in a leveling-rod assembly and a doctor blade assembly, even minimal deflections in the backing lath cause substantial deviations in the coat weight applied to the surface of the base web. A local deflection of the backing lath as small as about 0.075 mm causes a change of 1 to 2 g/m2 in the coat weight. Today, backing laths are generally made by machining from tin-bronze material. As the ultimate yield strength of tin bronze is low, forces imposed thereon from the machining process, the profile control screws and thermal expansion can readily subject the backing lath to permanent deformations that are difficult to compensate for however close to each other the control screws are placed.
Due to the relatively high Young""s modulus of tin bronze, a lot of force is required to bend such a backing lath, which means that the profile control screws must be placed maximally tightly pitched. However, rather small screws must be used to permit such a close spacing of the profile control screws.
Furthermore, tin bronze has a relatively high thermal expansion coefficient, whereby thermal expansion of the backing lath may cause deviations in the coat profile, particularly in the last coaters of a coater station and in coaters equipped with a so-called edge bead removal system that blows hot steam behind the doctor blade.
It is an object of the present invention to provide a novel type of doctor blade assembly capable of overcoming the above-described problems.
The goal of the invention is achieved by virtue of making the body of the backing lath from a fiber-reinforced composite material that has a high tensile strength, a low Young""s modulus and suitable thermal expansion coefficient. The body of the backing lath can be surfaced with a coating material that is resistant to wear and environmental attack.
The invention offers significant benefits.
The Young""s modulus of a backing lath made from a composite material is only about one-tenth of the Young""s modulus of tin bronze, thus facilitating easier bending of the backing lath. Resultingly, the distance between the profile control screws can be made larger than in the prior art. Furthermore, a backing lath made from a composite material has no detectable ultimate yield strength, which means that the strip will not exhibit any permanent deformations due to yielding. Moreover, the tensile breaking strength of a composite material is manifold as compared to that of tin bronze. A backing lath made from a composite material is also free from permanent dimensional changes caused by thermal expansion. Additionally, the thermal expansion coefficient of the backing lath can be modified by proper alignment of fibers in the composite material. Herein, the thermal expansion coefficient of the backing lath is advantageously made equal to that of the framework of the doctor blade assembly, whereby it is possible to reduce the stresses imposed on the assembly from thermal expansion. A backing lath according to the invention can be made by pultrusion that is a dimensionally accurate method and offers low manufacturing costs once the investment in the molding die is covered. Additionally, the geometry of the backing lath can thus varied in a manner that is extremely difficult or even impossible to achieve by conventional machining techniques.